THE TRANSIT 81 



set up on the rear point, if the telescope is revolved before 

 the pointing is made and the bearing taken. 



To Measure a Horizontal Angle. Set up the instru- 

 ment, center it by means of the plumb line over the vertex 

 of the angle required, set the zeros of the two plates to- 

 gether, clamp them, and turn the telescope toward one of 

 the points, making the final adjustment by means of the 

 lower tangent screw. Then loosen the upper clamp, turn 

 toward the other point, clamp again, and set finally by the 

 upper tangent screw. Read the angle turned by means of 

 the vernier. If the instrument has two verniers, both may 

 be read and the average taken. 



Measurement by Repetition. A more accurate meas- 

 urement may be had by turning the angle several times, tak- 

 ing the final reading, and dividing it by the number of 

 times the angle has been turned. If the final reading is 

 about 360, possible errors in the graduation of the instru- 

 ment will have no effect on the angle read, and if later the 

 telescope is inverted and the angle turned in the opposite 

 direction from the first turning, other sources of error will 

 have been eliminated. The exact program for an obser- 

 vation of this kind is as follows : 



a. Telescope direct. 1 



1. Clamp plates on zeros, and set on left station. Clamp 

 below. 



2. Unclamp above and set on right station. 



3. Unclamp below and set on left station. 



4. Unclamp above and set on right station. 

 Continue until the desired number of turnings have been 



made, when the final reading may be taken. 



b. Telescope inverted. 



1. Clamp plates on zeros and set on right station. 

 Clamp below. 



2. Unclamp above and set on left station. 



3. Unclamp below and set on right station. 



4. Unclamp above and set on left station. 



Continue for the same number of turnings as before 



1 That is, with the level tube underneath the telescope. 



